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Monday, February 11, 2013

Pilgrim Nuclear Power Station - Blizzard-related Outage

Over the weekend, Pilgrim Nuclear Power Station experienced what is referred to as a LOOP or Loss Of Offsite Power event and shut down. Many people and websites have questioned the events and/or safety of the plant. I'll describe what occurred, and include NRC Event Reports to help detail the situation.

Above, illustration of Pilgrim Station from Boston Edison brochure "About Pilgrim," 4/81. Pilgrim was ordered by Boston Edison in 1965. The plant was designed and built by Bechtel Corporation; construction began in late 1967, with initial criticality June 16, 1972 and first electric power generated July 19, 1972. The plant was declared commercial on December 9, 1972.

What actually happened was that storm-related damage to the electric distribution grid suddenly caused the plant to have nowhere to send the electric power it was generating, and of course get no outside power for any plant services to power anything. The result was a trip of the turbine generator, a reactor trip (scram) and shutdown of the plant. The EDG's or Emergency Diesel Generators started automatically to provide power.

Because of the LOOP event, the facility is required to declare "Notification Of Unusual Event." This happened when the grid connection was lost; the situation was rectified, but then further storm related events caused the plant to lose power connection to the outside again and a second LOOP event occurred.

It's important to note that the shutdown of the plant went as designed, and that the reactor is safely in cold shutdown. Time and again, this is the chain of events - even though the anti-nuclear web pages continue to predict disaster at U.S. nuclear plants related to weather, the plants consistently operate safely even in the very worst possible weather, including flooding, tornadoes, hurricanes and blizzards. There have also been earthquakes. Look here for a great history compiled by NEI showing some of the serious weather events that U.S. nuclear plants have made it through over the years.

Entergy's Pilgrim Power Plant site is active, and their Pilgrim account on Twitter has re-awakened; you can find them on Twitter using the handle @PilgrimNuclear.

Some people in the pro-nuclear world have asked if such a series of events that happened this weekend should be emphasized -- in other words, are they really newsworthy? Until the public finally realizes that nuclear power plants have historically weathered every single major environmental catastrophe in the United States without serious incident, perhaps such events are news. Eventually people will tire of getting all worked up over nothing and stop auguring disaster when severe weather events arrive.

How can they do that? They can do that because of the incredibly competent people who operate, maintain, supervise and (via NRC) regulate the nuclear plants. This is not to say that the public has no stake, and no say; it is to say that the level of hyberbolic concern is not justified. Until that time, those of us who report on such things will have to keep reporting and communicating clearly, honestly, and frequently.

Below, for completeness, I'll reproduce the text of the two Event Notifications that are presently available from the NRC. If you read through these, you'll see the actual events and times, and that the plant and staff have responded admirably.

-----NRC Event Notifications follow-----

(1) 02/08/2013 2250 HRS

UNUSUAL EVENT DECLARED DUE TO LOSS OF OFFSITE POWER

"Pilgrim Station
scrammed on a loss of offsite power. All systems performed as designed. Groups
I, II, VI went to completion. Reactor Core Isolation Cooling (RCIC) is injecting
to the vessel controlling level. High Pressure Coolant Injection is in pressure
control and slowly cooling down. Offsite power was lost multiple times. The
Startup Transformer has been declared inoperable. The Unusual Event was declared
under EAL SU 1.1 based on loss of offsite power greater than 15 minutes [at 2200
EST]."

The licensee originally experienced an automatic reactor scram at
2117 EST due to a load reject with a turbine trip/reactor scram due to loss of
power. Offsite power availability has been fluctuating in and out to the site.
The licensee states that all systems are functioning as required. All rods fully
inserted and the reactor is stable in Mode 3. Both Emergency Diesel Generators
are providing power to the safety related buses. The loss of offsite power is
believed to be weather related.

The licensee has notified the State and
local authorities and the NRC Resident Inspector.

Pilgrim terminated the Unusual Event and has transitioned to recovery
effective at 10:55 AM on 02/10/2013. Offsite power has been restored to
safety-related and non-safety-related electrical buses through the station
Startup Transformer via a single 345 KV line. The other two offsite power
sources remain out of service. The emergency diesel generators have been secured
and are in standby. Residual heat removal is in shutdown cooling mode
maintaining the reactor in cold shutdown. Fuel Pool Cooling is in service with
fuel pool coolant temperatures trending down.

"On Sunday, February
10, 2013 at 1405 [EST], with the reactor at 0% core thermal power, all control
rods fully inserted, and in cold shutdown conditions, the plant experienced a
loss of off-site power. With Pilgrim Station aligned to off-site power via the
start-up transformer (SUT), a fault on the 'B' phase of the SUT was experienced
due to suspected falling ice striking the phase's insulator. This resulted in
the tripping of the feeder breaker, ACB-102, and the loss of power to 4160 KV
buses A1 through A4. Emergency Diesel Generators (EDGs) 'A' and 'B' auto-started
as designed and are powering emergency buses.

"The loss of off-site
power resulted in de-energization of both Reactor Protection System (RPS)
channels resulting in a reactor scram signal and isolation of shutdown cooling.
At 1418, shutdown cooling was returned to service. All other plant systems
responded as designed. Station personnel are in the process of establishing
back-up power in accordance with plant procedures.

"The following press
release was made at 1715 hours: 'Offsite power to Pilgrim station was
interrupted this afternoon. The plant is in a cold shutdown condition and
Pilgrim's diesel generators are providing power to the site. There is no worker
or public safety concern. Plant personnel are troubleshooting the cause of the
interruption.'

"This event had no impact on the health and/or safety of
the public.

"The NRC Senior Resident Inspector has been notified."

--------------------

As always, you can keep up with the latest by following Atomic Power Review on Twitter - look for me there using the handle @atomicnews for updates on all things nuclear.

Mark, most nuclear plants don't have the ability to power down to low levels in the event of a 100% generator load reject. It is theoretically possible to do so but would require very costly modifications to major plant components. Pilgrim actually does power its own systems using the power it generates when the plant is online, when it is offline the systems are powered directly by the grid. But the snowstorm knocked out all of the 345kv lines which caused both a loss of offsite power and interrupted the electrical output of the plant to the grid causing the 100% load reject. Some of the newer reactor designs actually will have the ability to quickly power down to low levels to power plant house loads even with a 100% loss of grid load, but as far as I know none of the existing plants in the USA are designed to be able to do this. It's mainly a limitation of the plant's condenser. You'd have to dump all the steam being generated by the reactor for the duration of the time it takes to reduce the reactor power to the appropriate level. At Pilgrim (and all current plants) the condenser tubes are not designed to be able to handle that. Hope this answers your question.

When you lose load, things tend to happen very quickly. Loss of offsite load means the power has no where to go, which if unchecked would cause overspeed damage to the turbine. So the safety system turns off the reactor and also activates the main steam isolation valves to dump steam. I don't think there is time to do a controlled runback of power to a lower level that could be used to run plant systems. There is nothing inherently dangerous about doing this. You have to do a normal retsrat after the shutdown when the load is reconnected to the plant, but operators know how to do it.